1. Field of the Invention
This application relates to the field of surface coatings and more particularly to the field of surface coatings using chemical vapor deposition.
2. Description of Related Art
Chemical Vapor Deposition (CVD) coating relates to a technique in which gaseous substances are brought into a chemical reaction with the surfaces of other materials, for example tools for metalworking, under certain pressure and temperature conditions, so that the molecules contained in the gases are deposited on the surfaces of the other materials as a result of thermal reactions to form a coating. CVD coating is generally known in the prior art.
The design of such a coating system is described for example by Hegi in the journal "Metalloberflache," Vol. 37, 1983, No. 4, pages 166-168. In systems of this kind, the coating of the work pieces takes place in a reactor chamber, in this case with a rising reaction gas. In this coating system the gases, in other words the reaction gas mixture, is introduced through a central tube in such fashion that it rises in the reaction chamber, i.e. in the vicinity of the work pieces, with the process gases being drawn off through an exhaust pipe located above and then conducted to an exhaust-gas-neutralizing unit outside the reaction chamber.
Likewise, in the system known from U.S. Pat. No. 5,503,875, the reaction gas mixture is introduced into the reaction chamber, but laterally in this case, in such fashion that it rises. In addition, systems are known for example from U.S. Pat. No. 5,441,570 in which the reaction mixture is introduced from above, in other words so that it falls.
In addition, a device is described in DD 111 935 B1 in which the reaction gas mixture is introduced through a central tube that has gas outlets distributed over its height. In the reaction chamber, the reaction gas mixture always flows radially from the inside to the outside, in other words perpendicularly to the lengthwise axis of the reaction chamber.
The known versions mentioned here as examples show that in CVD systems, the flow conditions of the gaseous reactants are important criteria for the function, since when the reaction gas mixture is introduced into the reactor, the concentration of components that are capable of reacting (for example TiCl.sub.4, CH.sub.4, N.sub.2, etc.) necessarily decreases continuously in the flow direction. As a result, the layer-formation conditions also change continuously and the work pieces that are located in the reactor further away from the point of introduction, for example, are coated to a lesser degree or are not coated at all. In addition, the adhesion conditions change, as does the chemical composition, and possibly also the crystal alignment, the topography, etc. and thus the application-relevant quality level of the coatings decreases rapidly along the flow direction. This decrease is often unacceptable, especially in areas located more remotely from the point of introduction, in other words, in the vicinity of the exhaust gas location.
As a result of the difficulties discussed above, the length or height of reactors and hence the useful reaction space are limited. Thus, for example, hydrochloric acid (HCl), which is highly corrosive and is produced at a coating temperature of 1000.degree. C., will etch the work piece surfaces to an unacceptable degree further away from the point of introduction, while the desired layer formation takes place properly in the area near the point of introduction but may not take place further away from the point of introduction.
Since the decrease in concentration varies as a function of the gas mixture and type of coating, i.e. the layers are produced at different rates, in multilayer coatings (for example TiC--TiCN--TiN, with TiN as the outermost cover layer) a layer thickness distribution takes place under certain conditions such that the desired multilayer coating is not present at all on work pieces that are more remote from the point at which the gas is introduced. For example, there may instead be only a thin (and possibly even nonexistent) TiN layer on an etched work piece surface which is unacceptably roughened by etching. Layers of this kind are of unacceptable quality so that work pieces thus damaged may have to be discarded.
In addition, material transfers in the flow direction that are unsatisfactory because of disturbing values caused by decomposition can occur in the flow direction which result in application-relevant problems of coating adhesion as the size of the reactor chamber increases.
Another problem is posed by so-called Luv-Lee effects which cause different layer thicknesses depending on whether the surface to be coated is subjected to a frontal flow or whether this surface is on the side that faces away from the stream directed against it.
It is desirable to provide a method and a reactor system for CVD surface coating in which the disadvantages listed above are avoided, in other words the extremely important averaging of the reaction behavior along the flow to promote exact process control, layer quality, improvement of multilayer coatings, process yield, reactor upscaling, and cost reduction are achieved.